The present invention is directed generally to optical systems. More particularly, the invention relates to optical systems including optical amplifiers and methods for use therein.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing and communicating information has accelerated demand for high capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexed (“WDM”) transmission systems to increase the capacity of their optical fiber networks to meet the increasing demand.
In WDM transmission systems, distinct wavelength ranges that are useful for transmission through a transmission medium are allocated to carry separate information streams simultaneously within the medium. Analogously, distinct frequency ranges can be allocated to carry separate information streams in frequency division multiplexed (“FDM”) systems. The wavelength/frequency ranges of WDM, FDM, and other systems carrying multiple information streams are often referred to signal wavelengths/frequencies, or signal channels. The ranges are characterized by a center wavelength/frequency, which is typically the mid-point of the wavelength/frequency range. The ranges also may be characterized in other manners, such as the wavelength/frequency of maximum power or a relative to reference wavelength/frequency.
In WDM systems, signal channels are transmitted using electromagnetic waves within the distinct wavelength ranges in the optical spectrum, typically in the infrared wavelength range. Each signal channel can be used to carry a single information stream or multiple information streams that are electrically or optically time division multiplexed (“TDM”) together into a TDM information stream.
The pluralities of information carrying wavelengths are combined into a multiple channel, “WDM”, optical signal that is transmitted in a single waveguide. In this manner, WDM and other multiple channel systems can increase the transmission capacity of space division multiplexed (“SDM”), i.e., single channel, optical systems by a factor equal to the number of channels in the multiple channel system.
The development of optical amplifiers greatly reduced the cost of optical systems, particularly multiple channel systems. The ability of optical amplifiers to amplify multiple optical signals simultaneously essentially eliminates the need and associated cost to separate and repeat each channel electrically and/or optically merely to overcome signal attenuation.
Optical amplifiers can be deployed either as distributed amplifiers or localized amplifiers. Both types of amplifiers can be used in a system to provide gain to overcome various losses in the system, such as fiber and component losses.
A distributed amplifier is designed to employ a portion of the transmission medium, typically optical fiber, as the amplifying medium, such that the amplification is distributed along the transmission fiber. The distributed amplification has the effect of lowering the net loss in the fiber, which improves the transmission performance, i.e., reach, of an optical transmission system.
A localized amplifier, also referred to as a lumped, concentrated, or discrete amplifier, is located at a discrete point in the network. Localized amplifiers are desirable, because they can provide high gain in a localized portion of the network to overcome high fiber and/or component losses in the network.
In some system configurations it is desirable to deploy both distributed and localized amplifiers, sometimes referred to as hybrid amplifiers, in order to meet the performance requirements of a system. The most common configurations employ distributed Raman amplifiers along with localized erbium doped fiber amplifiers (“EDFAs”), although localized Raman and other non-linear amplifiers and distributed erbium or other doped fiber amplifiers can be deployed.
A limitation in multiple channel transmission systems, including WDM systems, is that the performance of signal channels being transmitted through the system can vary significantly over the wavelength range used to transmit signal channels through the system. The use of multiple amplifier types, such as distributed and localized Raman and erbium amplifiers, in the network, while improving overall system performance can exacerbate the non-uniform system performance. The exacerbation is due to different amplifiers having different performance characteristics, such as gain and noise figure over a signal channel wavelength range, which can negatively impact uniform performance over the wavelength range.
The varying degradation mechanisms can result in diminished system performance over at least a portion of the signal channel range. The systems can be operated in a number of mode such as a uniform performance specification mode across the signal channel wavelength range or in a mode with varying performance specifications across the signal channel wavelength range. Either of these operational modes is often less than desirable. The former mode results can result in the added expense associated with premature regeneration of better performing signal channels to maintain uniform system performance. The latter mode can place significant constraints on the network planning, provisioning, and operations, because signal channels must be matched to individual circuits based on the length of the circuit and the performance of the signal channel in a particular portion of the network.
Most designs merely focus on making the performance of each amplifier type in the system operate as uniform as possible or conforming some other desired performance variation. The resultant variation is system performance is merely addressed by placing constraints on the operation of the system, as described above. Some newer amplifier designs have been developed that specifically attempt to address the variations in noise figure and/or gain variations in an amplifier, such as in U.S. Pat. No. 6,356,383, which is incorporated herein by reference.
In general, the problem remains that noise figure and/or gain variations in the performance of optical amplifiers in multiple channel transmission systems limits the overall system performance. As such, there is a continuing need to improve optical amplifiers, amplification methods, and optical systems employing such optical amplifiers to provide for higher capacity and longer distance transmission systems.